Several solder containing compositions are known and used in the electronics industry. These solder containing compositions usually comprise a powdered metal alloy, fluxing agents, binders, and solvents for the binders. The most prevalent solder containing compositions comprise lead-tin (Pb--Sn) alloys and a fluxing agent. The fluxing agent is used to remove oxides that naturally form on the surface of the alloy powder, and the binder is used to hold the fluxing agent to the solder powder and/or provide a controlled amount of slump for those compositions which are intended to be screen printed. When these solder containing compositions are heated upon reflow, the solvents boil off as a vapor, and the binders and fluxing agents decompose, generating both gaseous byproducts and non-volatile residue. These solder containing compositions are often used to electrically connect the terminals of electrical components and integrated-circuit (IC) chips to printed circuit boards.
Another material used to make such connections are metal-filled epoxy compositions. Copper or silver particles, usually less than 50 .mu.m in size, are used with an epoxy resin and curing agent. The particles usually comprise a high weight percentage of the composition, usually greater than 75% by weight. Upon heating, the epoxy cures to immobilize the metal particles in an electrical network which makes the desired electrical connection. Epoxy resins can include branched polymers having two or more epoxide functional groups. The curing agent such as an anhydride reacts with the epoxide group to form a polymer network. The curing rate of an epoxy can be increased by increasing the number of available reactive epoxy functional groups per resin molecule. One way of characterizing the number of epoxy groups per resin molecule is to measure the number of epoxy groups per weight of resin. This can be characterized as the epoxy "equivalent weight" of the resin. Epoxy functional resins having relatively higher rates of cure usually have equivalent weights of less than 200.
Other approaches for connecting electrical components to circuit boards include the so-called "anisotropic conductive film" (ACF) and the "anisotropic conductive material" (ACM). These approaches are similar in that they both use large spherical or cylindrical conductive bodies which are distributed in a thermosetting or thermoplastic polymer. They are different in that the ACF film is preformed in the form of a bonding sheet while the ACM material is a spreadable liquid. The conductive bodies may comprise metal or metal-coated polymeric materials, which are resilient and elastic. To use the ACM or ACF, the ACM or ACF is placed between an electrical terminal and a corresponding pad and is heated with the terminal being pressed against the pad. The terminal makes electrical contact to the pad through at least one conductive body, with excess polymer being squeezed away from the top and bottom of the body by the applied pressure. Upon heating, the polymer cures and changes from a liquid to a solid. Upon cooling, the polymer shrinks in the vertical direction, and thereby applies a contractive force between the terminal and pad, which in turn maintains pressure on the conductive bodies between the terminal and pad. The term "anisotropic" arises because the conductive bodies only conduct electrical current vertically between the terminal and pad, rather than in all directions, as would be the case with a metal-filled epoxy.
The electrical connections provided by metal-filled epoxies, ACFs, and ACMs usually have higher electrical resistances than those provided by alloy solders (which, by nature, make metallurgic bonds to the terminal and pad). ACFs and ACMs are also known in the art to have long term reliability problems.